The present invention relates to timing chain art. It finds particular application in conjunction with a camshaft drive for a V-block engine and will be described with particular reference thereto. However, it should be appreciated that the present invention may also find application in conjunction with other types of chain drive systems and applications where reduced engine noise is advantageous.
In a typical overhead camshaft engine, V-type as well as horizontally-opposed, a separate camshaft chain drive is often utilized for each cylinder bank, and the two drives often have a substantially identical chain drive geometry for each bank. This drive configuration requires the utilization of two coaxial crankshaft sprockets. Alternatively, a single crankshaft sprocket having two sets or rows of teeth can be utilized. The meshing for the crankshaft sprockets is in phase or, in other words, occurs at substantially the same instant for any geometry in which the bank angle is an integer multiple of the tooth angle. It is known that simultaneous roller-sprocket meshing engagement results in higher chain drive noise levels.
FIGS. 1 and 2 illustrate the geometry for a typical V-type engine camshaft chain drive system 8 having an exemplary bank angle .alpha. of approximately 60.degree.. A right-hand cylinder bank drive 10 includes driven intake camshaft sprocket 12 and exhaust camshaft sprocket 14, crankshaft drive sprocket 16, and chain 20. Likewise, a left-hand cylinder bank drive 22 includes driven intake camshaft sprocket 24 and exhaust camshaft sprocket 26, crankshaft drive sprocket 30, and chain 32. In the described embodiment, the camshaft sprockets 12, 14, 24, 26 are 36-tooth sprockets. The crankshaft sprockets 16, 30 are secured to a crankshaft 34 in an in-line or co-axial configuration. The crankshaft 34, and hence sprockets and chains rotate in the direction of arrow 36.
A first chain guide 40 positions or otherwise guides a taut strand 20a of the chain 20 along a fixed arcuate path between the intake sprocket 12 and the crankshaft sprocket 16. Likewise, a second chain guide 42 positions or otherwise guides a taut strand 32a of the chain 32 along a fixed arcuate path between the exhaust sprocket 26 and the crankshaft sprocket 30. The chain paths for each bank 10, 22 are substantially identical, with both taut strands 20a, 32a having the same curvature (i.e. radius) and tangent offset 43.
The tangent offset is a measure of how far a midpoint of the taut strand 32a is offset from a line 44 tangent to the outermost edges of the chain links that are wrapped about each sprocket 26, 30. Thus, the tangent offset of the taut-strand 20a is substantially equal to the tangent offset of the taut-strand 32a. The bank drives 10, 22 also include conventional tensioners and tensioner arms that act on the chains to produce the required chain strand control.
As shown more clearly in FIG. 2, the sprocket teeth of the crankshaft sprockets 16, 30 are conventionally in phase (i.e. no circumferential offset therebetween). The taut-strand rollers of chain 20 successively mesh or otherwise engage with the crankshaft sprocket 16 at a fixed location A, and the taut-strand rollers of chain 32 successively mesh with the sprocket 30 at a fixed location B. The meshing engagement point B is offset from the meshing engagement point A by an angle substantially equal to the bank angle .alpha. or, in this case, 60.degree.. Although the roller-sprocket meshing points A, B are offset, roller-sprocket meshing for both banks occur at substantially the same instant in time which, as previously mentioned, results in higher chain drive noise levels.
One solution to the problem of higher chain drive noise levels is to shift or phase the roller-sprocket meshing for the crankshaft sprocket of one bank with respect to its counterpart for the opposite bank so that the roller-sprocket collisions do not occur at substantially the same instant in time. FIG. 3 illustrates a known chain drive arrangement wherein the sprocket teeth of the coaxial crankshaft sprockets 16, 30 are phased to achieve bank-to-bank roller-sprocket meshing phasing of one-half pitch.
In other words, the 18-tooth (where N=18) crankshaft sprockets--which have tooth angles of 360.degree./N or 20.degree.--have a bank-to-bank meshing phasing of 10.degree.. Thus, a roller 46 of chain 20 is fully engaged with the sprocket 16 at engagement point A, while a roller 50 of chain 32 is still 10.degree. from meshing with the sprocket 30. The roller 50 does not fully engage with the sprocket 30 until the crankshaft sprockets 16, 30 rotate an additional one-half pitch or 10.degree. in the direction of arrow 36 to engagement point B. Thus, the engagement impacts for each bank are staggered in time and the resultant chain drive noise is reduced.